Question

A car traveling 100 km/h is 200m away from a truck traveling 50 km/h (in the same direction). Assuming constant braking acceleration, what is the minimum deceleration the car must have if it is not to hit the truck?

Can someone hint me on how I can start this problem? Thanks

Answer 1

Car Speed=100km/h=100000m/3600s=27.8m/s

Truck Speed =(50/100) * 27.8=13.9 m/s.

V^2 = Vo^2 + 2a*d
a = (V^2-Vo^2)/2d
a = (0-(27.8-13.9)^2)/400 = -0.483 m/s^2

NOTE: The effective speed of the car is less than the actual speed, because the truck is moving forward.

Answer 2

Correction: The speed of the car does not have to be reduced to zero, because
the truck is moving forward. It must be reduced to 50 km/h(13.9m/s).

a = (V^2-Vo^2)/2d
a = (13.9^2-27.8^2)/400 = -1.45 m/s^2.